1. Field of the Invention
The present invention relates to a technology for stacking sheets in an image forming apparatus.
2. Description of the Related Art
With a wide spread of an image forming apparatus, such as a copier, a printer, a facsimile (FAX), and a digital multifunction product (MFP), there are cases where a large number of sheets are discharged from the image forming apparatuses. The sheets include a recording paper, a transferring paper, an overhead projector (OHP) transparency, a sheet-type recording media, etc. A typical discharge tray that receives discharged sheets is configured to move up and down for aligning the stacked sheets according to an amount of the discharged sheets stacked thereon. If a large number of stacked sheets are removed from the discharge tray, a distance between a discharge port from which the sheet is discharged and a top of the sheets stacked on the discharge tray is increased. Then, when the next sheet is discharged, the sheet falls from the discharge port onto the discharge tray by the distance, and may disadvantageously cause the stacked sheets to be misaligned. To prevent such cases, the discharge tray is required to move up immediately when a large number of stacked sheets are removed from the discharge tray.
Most conventional discharge trays requiring a large load use a direct current (DC) brush motor as a driving source. In DC motors, a rotational speed is inversely proportional to an amount of the load. Therefore, in most cases, a control of the driving speed is not performed when using the DC motor as the driving source.
In the discharge tray without the control of the driving speed, a moving-up time is variable depending on the number of the stacked sheets, which causes an adverse effect in performance of sheet processing. Various technologies for solving the problem have been disclosed for far.
For example, Japanese Patent Application Laid-open No. 2005-170578 discloses a technology to correctly move the discharge tray up to a stand-by position in a short time after a large amount of stacked sheets is removed from the discharge tray, thereby maintaining productivity of the image forming apparatus. Japanese Patent Application Laid-open No. 2005-170578 discloses a post-processing apparatus that receives a sheet from the image forming apparatus, post-processes the sheet, and discharges the post-processed sheet onto the discharge tray. The post-processing apparatus includes a first detecting unit that detects the stand-by position of the discharge tray; a second detecting unit that detects a position of the top of the stacked sheets, selects a switching position at which the moving speed of the discharge tray is to be switched according to the detected top-surface position; a driving unit that moves the discharge tray at the variable moving speed; and a post-processing control unit that controls the driving unit at a variable speed. The post-processing control unit controls the moving speed of the discharge tray via the driving unit based on a result of the detection by the second detecting unit.
Japanese Patent Application Laid-open No. 2000-177911 discloses a sheet stacking device including a plurality of sheet stacking units with a simple and cost-reduced mechanism for detecting a paper-full state etc. The sheet stacking device includes a movable first sheet stacking unit provided, as a unit, with an end fence for aligning a trailing end of the stacked sheets in a sheet discharging direction; a full-state detecting unit that detects whether the first sheet stacking unit supports a maximum amount of the sheets; a movable second sheet stacking tray capable of receiving a large amount of sheets; and a height detecting unit that detects a position of the top of the sheets stacked on the second sheet stacking tray. The first sheet stacking tray and the second sheet stacking tray moves independently. The full-state detecting unit and the height detecting unit share at least a relevant part thereof.
In the technology disclosed in Japanese Patent Application Laid-open No. 2005-170578, the switching position is set to the position of the top of the sheets stacked on the discharge tray. The object of this technology is just to increase accuracy of a stop position at which the discharge tray stops by decreasing the moving speed immediately before the discharge tray stops. The technology disclosed in Japanese Patent Application Laid-open No. 2000-177911 is related to the sheet stacking device including the sheet stacking units with the full-state detecting unit and the height detecting unit, both used for controlling up or down movement of the sheet stacking units, sharing at least a relevant part. That is, this technology is not directly related to control of up or down movement of the sheet stacking units.
If the DC motor is used as the driving source of the sheet stacking tray, the DC motor automatically decreases, from the nature of the DC motor as described above, the moving speed of the sheet stacking tray to the low value in a highly loaded state. However, the DC brush motor has disadvantages in a short lifetime and a high frequency of maintenance. Moreover, because the driving speed fluctuates according to the amount of the sheets stacked on the discharge tray, a complicated control system is required to align the stacked sheets with high accuracy.
In contrast, brushless motors have a long lifetime. The brushless motors have received attentions as the driving motor of the sheet stacking tray, recently. However, because the rotational speed of the brushless motor does not automatically decrease in the highly loaded state, it is necessary, if the brushless motor is used as the motor that moves the stacking tray up and down, to run the brushless motor at a high speed corresponding to a maximum load expected to be generated when the stacking tray supports the maximum amount of the sheets. Usage of the high-performance motor leads up-sizing and a cost increase of the motor as the driving unit. Moreover, the high-speed revolution leads an increase of noise.